In order to realize an increased recording density in magnetic recording devices, magnetic recording media utilized in the devices have tended toward increased track density resulting from narrower tracks. Moreover, the line density of the magnetic recording media has also tended to increase as a result of the use of higher frequency recording in recent years. For this reason, different kinds of thin-film magnetic heads with steep head magnetic field distribution have been proposed to cope with the higher density recording in the magnetic recording media and, at present, such magnetic heads are being employed.
For example, a thin-film magnetic head of the magnetism induction type has, as shown in FIG. 1, a lower magnetic core 3 and a magnetic gap-forming layer 4 on the substrate 2, a lower insulation layer 5 and an upper insulation layer 6 formed on the magnetic gap-forming layer 4, coil-formed electric conductive layers 7, the cross-section of which is expressed as it is formed between the magnetic gap-forming layer 4 and the lower insulation layer 5, and between the lower insulation layer 5 and the upper insulation layer 6, an upper magnetic core 8 formed on the magnetic gap-forming layer 4 and the upper insulation layer 6, and the protection layer 9.
The recording and playback characteristics of the thin-film magnetic head 1 are governed largely by the magnetic characteristics of the lower and upper magnetic cores 3, 8. For example, to enable the head 1 to be used in high frequency regions (.apprxeq.10 MHz), the magnetic permeability of the lower and upper magnetic cores 3, 8 must be large in high frequency regions. Accordingly, uniaxial anisotropy is used so that the track width in the thin-film magnetic head 1 becomes an easily magnetized axis, whereas a magnetizing process using a magnetizing rotation can be utilized when a magnetic field is applied perpendicularly to the track width during recording or playback. A permalloy NiFe alloy with a saturated magnetic flux density from 7,000 G to 8,000 G has been used conventionally for this type of magnetic core.
Furthermore, in magnetic recording field, it has been a growing trend to utilize media having a greater coercive force to raise the recording density, for which, because the saturated magnetic flux density of the conventional permalloy NiFe alloy is too low, Co-based amorphous alloys have been adopted rather than the former material. In the case of these Co-based amorphous alloys, the saturated magnetic flux density is about 12,000 G, which is sufficiently high to correspond to the coercive force of the magnetic recording media.
However, since magnetic core materials having high saturated magnetic flux density, such as a Co-based amorphous alloy, have an inherently strong uniaxial anisotropy, the anisotropic magnetic field, Hk, of such materials is so large that it causes the several problems, namely, because it is necessary to increase the operating frequency for the thin-film magnetic head 1 to achieve a higher frequency recording, the lower and upper magnetic cores 3, 8 must have a high magnetic permeability .mu. at their operating frequency. The magnetic permeability .mu. is expressed by the following equation, where a magnetic core material, which has a large anisotropic magnetic field, Hk, has its magnetic permeability .mu. reduced: EQU .mu.=Bs/Hk
where Bs is a saturated magnetic flux density.
Therefore, a method has been used conventionally to relax the magnetic anisotropy and reduce the anisotropic magnetic field Hk by heat treating the lower and upper magnetic cores 3, 8 and providing a rotating magnetic field application treatment under an atmosphere at a certain temperature, as disclosed in the Japanese Publication of examined patent application S59-35431.
However, the method of simply applying a magnetic field in an atmosphere at a certain temperature, as in this conventional anisotropy relaxing temperature, has such a small effect that, when the above magnetic materials are used, the magnetic permeability of the lower and upper magnetic cores 3, 8 reaches only about 2,500, causing a problem in that the magnetic head 1 cannot follow the higher frequency signals recorded on the magnetic recording media.
Furthermore, conventional methods of relaxing the magnetic anisotropy require a very bulky and expensive magnetic field application device. For example, the typical conventional production process for the thin-film magnetic head 1 builds several hundred elements collectively on one substrate via a photolithography technique, and cuts the substrate to produce individual thin-film magnetic heads 1. Therefore, many substrates having a diameter of 3 to 4 inches are heat treated while being subjected to a uniform magnetic field collectively. Hence the process requires a large heating furnace. Therefore, in order to apply a uniform magnetic field of about 1 kOe to each part of each substrate externally from the heating furnace, the magnetic field generator employed is a bulky and expensive piece of equipment, which causes problems during actual operations.
Given these problems, it is an object of the present invention to provide thin-film magnetic heads capable of high magnetic permeability. It is a further object of the invention to provide a method of manufacturing the heads that makes it possible to use a small and practical magnetic field generator by optimizing the conditions of applying an external magnetic field during magnetic anisotropy relaxing treatment.